1. Field of the Invention
The present invention relates to an arrhythmia detection system and method, and more particularly to an improved system and method for defibrillating the heart of a patient when the patient experiences life-threatening fibrillation.
2. Description of the Prior Art
In recent years, substantial progress has been made in the development of defibrillation techniques for providing an effective medical response to various heart disorders or arrhythmias particularly ventricular fibrillation which is characterized by an irregular ECG waveform. Earlier efforts resulted in the development of an electronic standby defibrillator which, in response to detection of abnormal cardiac rhythm, discharged sufficient energy, via electrodes connected to the heart, to depolarize the heart and restore it to normal cardiac rhythm. Examples of such electronic standby defibrillators are disclosed in commonly assigned U.S. Pat. No. 3,614,954 (subsequently, U.S. Pat No. Re. 27,652) and U.S. Pat. No. 3,614,955 (subsequently, U.S. Pat. No. Re. 27,757).
Past efforts in the field have also resulted in the development of implantable electrodes for use in accomplishing ventricular defibrillation (as well as other remedial techniques). In accordance with such techniques, as disclosed (for example) in U.S. Pat. No. 4,030,509 of Heilman et al, an apex electrode is applied to the external intrapericardial or extrapericardial surface of the heart, and acts against a base electrode which can be either similarly conformal or in the form of an intravascular catheter. Such electrode arrangements of the prior art, as disclosed in the aforementioned patent of Heilman et al, can employ independent pacing tips associated with either a base electrode or an apex electrode, or both.
Recent efforts also have resulted in the development of techniques for monitoring heart activity (for the purpose of determining when defibrillation or cardioversion is necessary), which techniques employ a probability density function for determining when ventricular fibrillation is present. Such a technique, employing the probability density function, is disclosed in U.S. Pat. Nos. 4,184,493 and 4,202,340, both of Langer et al.
In accordance with this latter technique of the prior art, when the probability density function is satisfied, fibrillation of the heart is indicated. However, recent experience has shown that, with one or more particular abnormal ECG patterns, the prior art probability density function detector, if not optimally adjusted, can be "triggered" not only by actual ventricular fibrillation, but also by some forms of high rate ventricular tachycardia, and low rate ventricular tachycardia as well, particularly in the presence of ventricular conduction abnormalities. Unlike ventricular fibrillation, such high rate and low rate tachycardias are characterized by regular R-waves occurring at generally stable rates. The possibility of such triggering in the presence of high rate tachycardia is acceptable because high rate tachycardia can be fatal if present at such a rate that sufficient blood pumping no longer is accomplished. However, triggering in the presence of non-life threatening, low rate tachycardia could be considered a problem. Therefore, it has been determined that there is a need for a system and method for distinguishing between ventricular fibrillation and high rate tachycardia, on the one hand, and low rate tachycardia, on the other hand.
It is worth noting that prior art implementation of the probability density function technique was, for a time, limited to "triggering" only in the presence of ventricular fibrillation. This was accomplished by adjusting the decision boundaries of the probability density function detector in a conservative way so as to "trigger" only upon occurrence of life-threatening ventricular fibrillation. However, it was soon realized that there existed situations in which it was desirable to take remedial action upon the detection of high rate tachycardia, as indicated by occurrence of a heart rate above a lower threshold level (for example, above 200 beats per minute). This was initially accomplished merely by adjusting the probability density function criteria so as to be "triggered" at the lower threshold level.
However, it was soon discovered that a problem existed in a detector with relaxed decision criteria, in that extraordinary types of ECG signals were capable of "triggering" the modified probability density function detector, even though neither ventricular fibrillation nor high rate tachycardia was present. Therefore, it has been determined that there is a need for an arrhythmia detection system and method which not only performs a probability density function analysis, but which also includes some technique for distinguishing between fibrillation and high rate tachycardia, on the one hand, and low rate tachycardia, on the other hand. Thus, the inventive system and method herein disclosed amounts to a "backup" technique by means of which high rate tachycardia is treated by issuance of a defibrillating shock to the patient, while low rate tachycardia is not so treated.